1. Field of the Invention
The present invention relates to a microwave oven having an automatic bread making function, for automatically performing a bread making process from the step of kneading bread ingredients to the step of baking the same.
2. Description of the Prior Art
A microwave oven having a function for mixing food has been proposed in Japanese Utility Model Unexamined Publication No. 55-71489. This microwave oven is configured with a casing removably attached to a turntable which is rotatably provided in a heating chamber, and a mixing blade provided within the casing removably attached to a fixed shaft passed through a rotary shaft of the turntable.
In the above described microwave oven, the casing attached to the turntable is disposed to be rotated while the mixing blade is stationary. Consequently, a subject to be heated may fly out of the casing due to centrifugal force if the casing is rotated at a high speed (for example 200 r.pm. which is relatively higher than the speed of an ordinary turntable). Accordingly, there has been a problem in that the speed of the turntable in ordinary microwave heating is limited to 5-6 r.p.m. At his speed, it is difficult to achieve sufficient mixing of the subject to be heated.
On the other hand, an automatic special purpose bread making apparatus has been proposed in Japanese Patent Unexamined Publication No. 62-111628. This apparatus comprises a baking chamber having a heater, a bread casing, or bread pan, arranged to be removably mounted in the baking chamber, a kneading blade provided on an inner bottom portion of the bread casing and arranged to be driven by a motor, and a controller for controlling the bread making process, according to a temperature of the bread casing as detected by a temperature sensor. The bread casing has a rectangular horizontal section and a rib substantially vertically projected from an inner wall of the break casing at opposing sides thereof.
In this configuration, the process of kneading bread ingredients is performed, with water and flour combined with each other to form numbers of nuclei first. Those nuclei are then successively combined with each other, forming a large lump of dough. In a rectangular bread casing, flour proximate a corner of the casing, where a larger interval between the inner wall of the bread casing and the forward end of the kneading blade exists, is not mixed. In order to mix the flour proximate to the corner of the casing with the lump of dough, the kneaded lump of dough must be moved into the corner and kneaded there. In the above configuration, at two of the four corners of the rectangular bread casing, the interval between the inner wall of the bread casing and the forward end of the kneading blade narrows as the kneading blade rotates. The lump of dough is further kneaded while it is stopped by the resistance of the rib, and the flour proximate the two corners is mixed into the lump of dough. In the rmaining two corners, on the other hand, there has been a problem in that the above-mentioned interval widens in front of the corners so that the lump of dough is rotated together with the kneading blade and the flour porximate these two corners is left unmixed.
Further, in a microwave oven disclosed in Japanese Utility Model Unexamined Publication No. 61-178296, a metal rotary shaft passing through a bottom surface of a heating chamber is supported by an upper bearing made of plastic and a lower bearing made of a sintered alloy. A so-called coaxial choke comprises a cavity portion, including a bearing holder and a pipe, which acts to prevent microwave energy from leaking to the outside. Further, a packing made of silicone rubber is interposed between the bottom surface of the heating chamber and the rotary shaft to thereby prevent water drops, or the like, from entering from the heating chamber.
In the above prior art microwave oven, the above-mentioned bearings and packing are always exposed to large microwave energy, while the energy can be prevented from leaking to the outside. Since the rotary shaft and the bottom surface of the heating chamber constitute a coaxial line, microwave energy tends to leak along the rotary shaft. The leaked microwave energy is reflected by the choke and returned to the heating chamber again, so that leakage of microwave energy to the outside via this route can be prevented. However, microwave energy passing through the upper bearing and the packing cannot be prevented. Particularly, since the inlet of the choke is disposed in the vicinity of the upper bearing and the packing, the upper bearing and the packing are located at a position extremely near the position of the maximum electric field of a standing wave due to the reflection. Being made of dielectric material, as well known, the upper bearing the packing generate heat due to dielectric loss if they are put in a large electric field. Further, if food remnants, spilled soup, or the like, are deposited in the vicinity of the packing, or if such food remnants or spilled soup enter into a gap caused by rotary abrasion between the packing and the rotary shaft, heat generation may be more pronounced, or sparks may result. Since these phenomena are dangerous, a solution to this problem is required. For the same reason, it is necessary to sufficiently protect the insulator in the vicinity of the terminals of the heaters passing through the wall of the heating chamber.